In general, shock absorbers of the type under consideration comprise a cylinder incorporating a hydraulic fluid and a piston head coupled to a piston rod extending from an end of the cylinder and reciprocable within the cylinder. A powerful compression spring surrounds the casing between the closed end of the cylinder and the extending end portion of the piston rod to bias the piston rod to its fully retracted position from the cylinder. When this assembly is coupled between the wheel of a motorcycle and the motorcycle frame, bumps and the like will urge the piston rod and piston towards the closed end of the cylinder, the hydraulic fluid passing from one side of the piston head to the other through small fluid bypass openings, thereby cushioning the shock. The exterior compression spring returns the piston head to the opposite end of the cylinder from which the piston rod extends.
In order to provide a controlled cushioning of the piston as it approaches the closed end of the cylinder, small pins are provided at the closed end which receive the fluid bypass holes in the piston head as it approaches the closed end thus effectively decreasing the cross sectional area of the fluid bypass openings so that the piston head experiences a greater hydraulic resistance and thus is more cushioned as it approaches the end of its travel.
In the foregoing type of design incorporating pins, a major problem is that of assuring that the pins are received in the fluid bypass holes when the piston approaches the closed end. Any rotation of the piston head, and/or piston rod relative to the pins which are normally secured to the closed end of the casing will result in misalignment and prevent the pins from entering the fluid bypass holes.
Shock abosrbers of the type under consideration also usually include a check valve which is spring loaded against the one side of the piston head facing away from the closed end to cover the outlets of the fluid bypass holes in the piston head. When subjected to shocks or movements, the fluid passing through the fluid bypass holes will force the check valve open. Thus, more refined hydraulic control can be realized by proper designing of the strength of the check valve spring. In addition, the check valve head itself may contain a small bleeder hole to permit movement of the valve head. However, there has been experienced difficulty in the reliability of the check valves themselves. In this respect, a desirable feature would be to be able to control the degree of pressure necessary to open the check valve as a function of its position along the cylinder.
Further desirable features would be to provide means for "fine tuning" of the dynamics of motion of the piston head through the hydraulic fluid in the cylinder.